There are image forming apparatuses such as printers, fax machines, and copiers. These image forming apparatuses form images including characters and symbols based on image data, and print the images onto recording media. In particular, electrophotographic image forming apparatuses can form highly precise images on plain paper at high speed, and are therefore widely used in offices. In electrophotographic image forming apparatuses, a heat fixing method is widely used. Specifically, toner resting on a recording medium is fused with heat, and pressure is applied on the toner, so that the toner is fixed onto the recording medium. The heat fixing method is preferable in that images are fixed at high speed and with high quality.
However, more than half of the power consumed by an electrophotographic image forming apparatus is used for heating the toner in the heat fixing method. In consideration of the measures for environmental protection in recent years and continuing, there are needs for a low power consuming (energy-saving) fixing device. Specifically, there is a need for a method of considerably decreasing the temperature at which the heat is applied to the toner for fixing the toner, or a method of fixing the toner without applying heat. An ideal method for saving energy is to fix the toner onto a recording medium without heating the toner at all (non-heat fixing method).
Patent Document 1 discloses a wet-type fixing method of fixing toner, which is an example of the non-heat fixing method. An oil-in-water type fixing agent is used in this method, which is made by dispersing and mixing an organic compound in water. The organic compound is soluble and can swell but is insoluble or hardly soluble in water. First, toner is applied onto a certain position on a material without being fixed thereon (unfixed on the material onto which the toner is to be fixed); the fixing agent is sprayed or dropped on the surface of the material; the toner melts or swells; and the material is dried, so that the toner is fixed on the material.
The wet-type fixing method disclosed in Patent Document 1 uses the oil-in-water type fixing agent in which the organic compound, which is insoluble or hardly soluble in water, is dispersed and mixed in water. Thus, when a large amount of the fixing agent is applied to the unfixed toner resting on a recording medium such as transfer paper (material onto which the toner is to be fixed), the recording medium absorbs the water included in the fixing agent. As a result, the recording medium is creased or curled. This obstructs the operation of consistently conveying recording media and at high-speed, which is a property required of an image forming apparatus. One approach is to use a drying device to evaporate the large amount of water included in the fixing agent in order to remove water from the fixing agent applied to the recording medium. However, this requires the same amount of power consumed by an image forming apparatus employing the heat fixing method.
There are several types of oil fixers that do not repel water-repellent unfixed toner. Such an oil fixer is obtained by dissolving, in an oil solvent, a material that causes toner to dissolve or swell. Patent Document 2 discloses a fixer in which the material for dissolving or causing swelling of resin comprising the toner is obtained by diluting (dissolving) a component such as aliphatic dibasic acid ester with a diluent (solvent) such as a nonvolatile dimethyl silicone. Patent Document 3 discloses a fixing solvent used in a fixing method of fixing an unfixed toner image formed by an electrostatic method onto an image receiving sheet, such that an undistorted, clear image is easily obtained. The fixing solvent is capable of dissolving toner, which fixing solvent is obtained by mixing 100 pts. vol. of a solvent that is compatible with silicon oil with 8-120 pts. vol. of silicon oil. This oil fixing solvent includes an oil solvent that is highly compatible with water-repellent unfixed toner. Therefore, the oil fixing solvent is capable of dissolving or causing swelling of the toner without repelling the water-repellent unfixed toner, and fixing the toner onto a recording medium.
Patent Document 1: Japanese Patent No. 3,290,513
Patent Document 2: Japanese Laid-Open Patent Application No. 2004-109749
Patent Document 3: Japanese Laid-Open Patent Application No. S59-119364
Patent Document 4: Japanese Laid-Open Patent Application No. 2004-109747
In the technology disclosed in the above Patent Documents, liquid is applied onto a layer of unfixed toner. However, problems arise as described with reference to FIGS. 13A, 13B. An application roller 1 is used as a contact-application unit for applying a fixer onto an unfixed toner layer 3 resting on a recording medium 2. When a fixer layer 4 on the application roller 1 is thinner than the unfixed toner layer 3 as shown in FIG. 13A, at the position where the application roller 1 separates from the recording medium 2, unfixed toner particles are pulled by surface tension on the surface of the application roller 1. The surface tension is generated by a liquid film of the fixer on the application roller 1. Accordingly, the toner particles adhere to the surface of the application roller 1 (also referred to as toner offset), and the image on the recording medium 2 is considerably disturbed. Conversely, when the fixer layer 4 on the application roller 1 is thicker than the unfixed toner layer 3 as shown in FIG. 13B, at the position where the application roller 1 separates from the recording medium 2, the surface tension of the liquid film of the fixer on the surface of the application roller 1 does not affect (move) the toner particles, because the amount of the fixer is excessive. Accordingly, the toner particles do not adhere to the surface of the application roller 1. However, a large amount of the fixer is applied on the paper (the recording medium 2), which takes a long time to dry, thus degrading the responsiveness of the fixing operation. Furthermore, a considerable amount of undried liquid remains on the paper (the recording medium 2), i.e., a damp feeling remains when the paper is touched with one's hand. Accordingly, with the method of applying a fixer with a roller, it is extremely difficult to enhance fixing responsiveness and reduce the amount of undried liquid by applying an extremely small amount of fixer on the toner layer resting on the paper, and at the same time prevent toner particles from adhering to the fixing roller. Even when a die coating unit, a blade application unit, or a wire bar application unit is employed as the contact-application unit, when an extremely small amount of fixer is used, the toner particles are caused to adhere to the contact-application unit due to surface tension.
As described above, in the conventional methods of applying the fixer with the contact-application unit, it is extremely difficult to achieve two properties at the same time; i.e., to enhance fixing responsiveness by applying an extremely small amount of fixer on the toner layer resting on the paper, and to prevent toner images from being disturbed by applying the fixer in a uniform manner.